There has been a conventional pressure transducer of the type employing a seal diaphragm and having such a structure that a pressure is transmitted to a diffusion strain-gage chip through a liquid, such as silicon oil.
Such a conventional pressure transducer is known by such as U.S. Pat. No. 3,697,919 issued on Oct. 10, 1972 in the title of "Semiconductor pressure transducer structure". In such a conventional pressure transducer, the strain-gage chip is isolated from an object to be measured and, therefore, the pressure transducer has excellent corrosion resistance. Further, since a pressure is applied from the surface of the gage, a stress acts on the gage bonding portion in the direction in which it is compressed and it is consequently possible for the pressure transducer to bear services under high pressures. In the structure of the type described above, reliability is affected by the manner in which the sealing of silicone oil or another type of oil is effectively maintained. To maintain the sealing of the oil, it is necessary to hermetically bond a seal diaphragm provided for the purpose of closing a recessed chamber in which the strain-gage chip is disposed. To obtain high reliability, a high quality welding technique and a material which produces scarcely any blowholes are required.
Accordingly, it is conventional practice to employ a vacuum melting material, such as SuS 304 or SuS 316, as a metal member in the pressure transducer and to conduct welding in a vacuum by means of electron beam welding, whereby the seal diaphragm made from a rolled material of SuS304 or SuS316 is hermetically bonded.
The above-described structure, however, involves an expensive welding device which has low operability in this application and a limited range of usable materials. For this reason, a pressure transducer having such a structure is disadvantageously high in cost. In addition, since the strain-gage chip is easily affected by heat, reliability is easily lowered.
From this perspective, to mount the pressure transducer on, for example, an automobile, it is necessary to discover a structure which enables the pressure transducer to be mass-produced at a reduced cost and permits high reliability to be maintained. These days in particular, there is an increasing demand for electronic control of oil pressures, and it is necessary to quickly develop a small-sized, highly reliable and low-cost pressure transducer for detecting the oil pressure for electronic controlled transmission and the oil pressure for engine control in an automobile, or for detecting the oil pressure in a hydraulic construction shovel.